Solenoid valves of the type mentioned in the introduction are known from the prior art. A solenoid valve of this type is by way of example the normally closed Bosch solenoid valve MV09OS with the TTNo. 1267691803 from the year 2014, which is illustrated in FIG. 1. The solenoid valve 1 comprises a solenoid actuator that comprises a solenoid coil (not illustrated), which can be energized, and also a pole core 2 and said solenoid actuator acts on an armature 3 that is arranged in an axially displaceable manner in the valve sleeve 5, 6. The valve sleeve 5, 6 can be configured as a two-part valve sleeve having an upper valve sleeve 5 and a lower valve sleeve 6. The valve sleeve 5 is positioned in the valve bush 7. A filter 8 surrounds the inlet in the valve sleeve 6. Furthermore, the armature 3 comprises a closing element 9 that is pushed against a valve seat 10 when the solenoid actuator is in the non-energized state. For this purpose, the armature 3 is held pre-stressed in the valve sleeve 5, 6. It is usual in order to produce the pre-stressing force to provide a compression spring 4, in particular a helical spring, which acts or rather is held in a pre-stressed manner between the pole core 2 and the armature 3. The compression spring 4 is supported at one end on the fixedly arranged pole core 2 and at the other end on the displaceable armature 3. For this purpose, the armature 3 comprises a recess and the compression spring essentially lies in said recess and is guided in said recess. In an alternative embodiment, the pole core 2 can also comprise a recess and the compression spring essentially lies in said recess and is guided in said recess—this embodiment is however not illustrated in FIG. 1. The portion of the compression spring 4 that protrudes beyond the recess is supported on the pole core 2 and extends from the end face of the armature 3 that is facing the pole core 2 up to the end face of the pole core 2 that is facing the armature 3 and the spacing with respect to one another forms the so-called working air gap. This working air gap determines the maximum possible displacement travel of the armature 3 and consequently the displacement travel of the solenoid valve 1. When the solenoid valve 1 is being energized, the air gap between the pole core 2 and the armature 3 is closed in that the armature 3 moves upwards until it impacts against the pole core 2. When the solenoid valve is being de-energized, the armature is moved by the compression spring 4 downwards against the valve sleeve 5,6 until the closing element 9 lies against the valve seat 10 and the valve is consequently closed. When the said solenoid valve is being energized, the magnetic force increases as the working air gap becomes smaller. This increasing magnetic force makes it more difficult to continuously adjust the solenoid valve 1. In order to improve the adjustability, it is known to provide a compression spring 4 that has progressive spring characteristic curves or also by way of example to provide an additional spring washer (not illustrated) that has a progressive spring characteristic curve.
The document DE102010040631A1 by way of example is known from the prior art. Said document describes a normally closed solenoid having a valve sleeve and a pole core is fixedly arranged in said valve sleeve and an armature that comprises a valve tip is arranged in an axially displaceable manner in said valve sleeve, wherein a helical spring acts between the pole core and the armature so as to push the valve tip into a valve seat. It is provided that a further helical spring is connected in parallel to the helical spring.
In the case of modern, so-called 1-box braking systems, the hydraulic assembly is to be screwed directly to the bulkhead of a vehicle. This means more stringent requirements with respect to the noise-vibration-harshness (NVH) behavior for all the components in the hydraulic assembly, the reason being that noises that occur by way of example as a result of switching solenoid valves of the hydraulic assembly are transmitted directly into the interior compartment as a result of this type of fastening. In the case of a normally closed outlet valve, it is possible by way of example for a clicking noise to occur during the opening procedure if when energized the armature that is mostly manufactured from steel impacts against the pole core that is likewise manufactured from steel.